Generally in heat treatment of heating a workpiece formed of steel in a controlled atmosphere, the atmosphere in the heat treatment furnace is collected and analyzed while atmosphere gas is introduced therein to control the atmosphere in the heat treatment furnace by adjusting the flow rate of atmosphere gas introduced into the heat treatment furnace (the supplied amount per unit time) based on the analyzed result. Accordingly, surface modification, suppression of surface degradation due to oxidation, or the like is achieved.
For example, in a gas carbonitriding process applied to a workpiece formed of steel, the atmosphere in a heat treatment furnace is controlled by introducing R gas and ammonia (NH3) gas into the heat treatment furnace at a constant flow rate, and controlling the carbon potential (CP) value in the heat treatment furnace based on the partial pressure of carbon dioxide (CO2) in the heat treatment furnace. It is difficult to directly measure the amount of nitrogen permeating into the surface layer of the workpiece during the carbonitriding process. In most cases, the amount of nitrogen permeating into the surface layer of the workpiece is controlled by adjusting the flow rate of ammonia gas that can be directly measured during a carbonitriding process, subsequent to empirically determining the relationship between the flow rate of ammonia gas and the amount of nitrogen permeating into the surface layer of a workpiece from past records of actual production in association with each heat treatment furnace.
The flow rate of ammonia gas is determined empirically, taking into account the mass, configuration and the like of the workpiece, based on the past records of actual production with respect to each heat treatment furnace. In the case where a workpiece of an amount or configuration whose records of actual production are not available is to be subjected to a carbonitriding process, the optimum flow rate of ammonia gas in the relevant carbonitriding process must be determined by trial and error. It is therefore difficult to render the quality of the workpiece stable until the optimum ammonia gas flow rate is determined. Moreover, since the trial and error must be carried out at the production line, workpieces that do not meet the required quality will be produced, leading to the possibility of increasing the production cost.
There is proposed a method of controlling the amount of nitrogen permeating into the workpiece by adjusting the undecomposed ammonia concentration (the concentration of residual ammonia gas) that is the concentration of gaseous ammonia remaining in the heat treatment furnace (for example, Yoshiki Tsunekawa et al. “Void Formation and Nitrogen Diffusion on Gas Carbonitriding” Heat Treatment, 1985, Vol. 25, No. 5, pp. 242-247 (Non-Patent Document 1) and Japanese Patent Laying-Open No. 8-013125 (Patent Document 1)), instead of controlling the flow rate of ammonia gas that varies depending upon the configuration of the heat treatment furnace, as well as upon the amount and configuration of each workpiece. Specifically, the undecomposed ammonia concentration that can be measured during a carbonitriding process is identified, and the flow rate of ammonia gas is adjusted based on the relationship between the undecomposed ammonia concentration and the amount of nitrogen permeating into the workpiece, which can be determined irrespective of the configuration of the heat treatment furnace and/or the amount and configuration of the workpiece. It is therefore possible to control the amount of nitrogen permeating into the workpiece without having to determine the optimum ammonia gas flow rate by trial and error. Therefore, the quality of the workpiece can be stabilized.
In addition, there is proposed a carbonitriding method allowing the permeating rate of nitrogen into a workpiece to be adjusted by employing, as a parameter, the γ value that is the carbon activity divided by the volume fraction of undecomposed ammonia (for example, refer to Japanese Patent Laying-Open No. 2007-154293 (Patent Document 2)). Accordingly, the quality of the workpiece can be further stabilized, and an efficient carbonitriding process can be implemented.
Non-Patent Document 1: Yoshiki Tsunekawa et al. “Void Formation and Nitrogen Diffusion on Gas Carbonitriding” Heat Treatment, 1985, Vol. 25, No. 5, pp. 242-247.
Patent Document 1: Japanese Patent Laying-Open No. 8-013125
Patent Document 2: Japanese Patent Laying-Open No. 2007-154293